I still gotta learn how to suggest patches to the core, this one would be much welcome, IMO that is... Just out of curiosity, what do you prefer: replacing the column or hack the DOM? And what would you consider flexible?
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brasofiloJul 29 '12 at 21:18

As I speculated, removing the icon column and putting a new column in its place would let us pick any image size
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Tom J Nowell♦Jul 29 '12 at 21:21

Expanding on brasofilo's solution I used the below to enlarge the thumbnails within the media uploader / gallery

Its far from perfect and needs more time spent - most noticeably with the active draggable area remaining the same as previous when resorting - but if you're dealing with 10's of similar images attached to a single post I found it a huge help as is.

Considering 3.5 is out within days and revamps the entire media uploader, does this work in 3.5RC3?
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Tom J Nowell♦Dec 7 '12 at 16:51

no idea sorry - this was just a quick and dirty fix using 3.4.2. Its 90% functional, the remaining 10% to make it perfect could take some time..
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zeniphDec 10 '12 at 1:20

The hook still works for plugins' upload popup windows, which don't have the new interface. :::: I tried your code, but unfortunately, it messes with the thickbox display, at least in 3.5. :::: I reduced it to the bare minimum. If it helps, update your Answer. Cheers!
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brasofiloJan 3 '13 at 0:24

What your asking isn't a "simple" fix. If you're serious about getting larger thumbs I'd start by looking at the wp-admin/class-wp-media-list-table.php file, or here in the PHPXref site (Around line 203). You can examine the code to see how it's generated and decide how to adjust it from there